The goal of this project is to understand how microtubule networks are regulated by molecular diversity at the level of tubulin proteins. Tubulin proteins assemble into dynamic microtubule polymers that form networks with associated motor proteins to organize the cell in diverse contexts. Defects in microtubule function are linked to numerous human diseases, including neuropathies and cancer. Understanding the mechanisms that regulate microtubule networks is therefore critical for understanding how defects contribute to the disease state. A major deficit in our understanding is how molecular diversity at the level of tubulin proteins -- mutations, isotype expression, posttranslational modifications -- lead to changes at the level of the network. My laboratory is now poised to address this deficit by overcoming two historical barriers in the microtubule field. First, we have developed a genetic model for manipulating tubulin proteins by directed mutation. Second, we extend our genetic model to complementary systems for analyzing how CTTs affect tubulin biochemistry, MT motor activity, and MT function in vivo. By applying these synergistic approaches, we can distinguish effects on intrinsic microtubule dynamics from effects on motors and other extrinsic regulators, and we can determine the consequences of these effects for a cellular network. In this project, we will determine the roles of carboxy-terminal tail motifs (CTTs) of ?- and ? tubulin. CTTs support electrostatic interactions with motors and other proteins at the MT surface, exhibit sequence diversity across tissue-specific tubulin isotypes, and are major sites of post-translational modification. We hypothesize that CTTs regulate network behavior by promoting microtubule dynamics and the activity of evolutionarily distinct classes of MT motors. We will test this hypothesis in three aims: 1) Determine how CTTs directly affect the assembly of tubulin proteins into microtubule polymers. 2) Determine how CTTs regulate evolutionarily conserved kinesin motors. 3) Define the molecular roles of CTTs in the assembly of a complex cellular microtubule network - the mitotic spindle. The results of this work will provide new insight into the molecular regulation of microtubule networks. Furthermore, these studies will set the stage for future investigations of the roles of tubulin posttranslational modifications, isotyp expression, and disease-associated alleles.